The present invention relates generally to power supplies and more particularly to a method and apparatus for detecting a reduction, such as an impending or actual loss, in input power or voltage to a regulated power supply.
It is well known in the art to supply apparatus such as microprocessors with electrical power from a supply and to provide some indication of inpending loss of input power/voltage (i.e., from the utility) so that data within the volatile portion of the microprocessor are not lost; that is, steps may be taken by the microprocessor to preserve this data. One such known power supply is what is termed a half wave, pulse width modulated (PWM) regulated switching power supply, sometimes called a flyback switching power supply. This type of power supply provides isolation through the use of a transformer. Isolation is required because of the voltage sensitivity of the load which, for example, operates at a nominal five-volt level. As such, any power line transient directly coupled to the load could easily result in the destruction or serious damage to that load.
Typically, in these power supplies, a detecting means is provided at the source side of the power supply and an alarm signal is coupled to the load side (e.g., the microprocessor) by way of appropriate isolating means such as an optical coupler. Such isolation schemes are generally satisfactory and have been well proven but do suffer from at least two major deficiencies. The first of these is that the source side voltage level (e.g., 120 volts) necessitates power level components which tend to be more expensive than signal level components. The second major disadvantage is that the isolating coupler is itself a source of potential failure as well as a fairly expensive item.